This project is designed to investigate a variety of environmental and hormonal mechanisms which are involved in regulating annual cycles of hibernation and body weight in three species of hamsters. All three species are photoperiodic and are long day breeders. Mesocricetus brandti and Cricetus cricetus are natural hibernators, while Phodopus sungorus displays daily torpor during the winter. In M. brandti and Phodopus the pineal gland is involved in mediating the effects of daylength on reproductive activity. The present studies will examine the effects of gonadal and pineal hormones and photoperiod, ambient temperature and food-availability on annual cycles of body weight and hibernation (daily torpor in Phodopus). In M. brandti, the site action of androgen in terminating hibernation will be studied by administering hormone via brain imp]ants. Patterns of secretion of LH, FSH, prolactin and testosterone will be examined in hibernating animals. The results will increase our understanding of how the environmental regulation of various annual cycles is physiologically mediated.